C：读取 function 未在断开的串行 USB 设备上返回错误

Question

I have a C application which opens a /dev/ttyUSB* port

    fd = open(portName.c_str(), O_RDWR | O_NOCTTY | O_NONBLOCK);
    options.c_cflag |= (CLOCAL | CREAD);
    options.c_cflag &= ~PARENB;
    options.c_cflag &= ~CSTOPB;
    options.c_cflag &= ~CSIZE;
    options.c_iflag &= (tcflag_t) ~(INLCR | IGNCR | ICRNL | IGNBRK);
    options.c_oflag &= (tcflag_t) ~(OPOST);
    options.c_cflag |= CS8;
    options.c_cflag |= CRTSCTS;
    options.c_lflag &= ~(ICANON | ECHO | ISIG);
    tcsetattr(fd, TCSANOW, &options);
    struct serial_struct kernel_serial_settings;
    if (ioctl(fd, TIOCGSERIAL, &kernel_serial_settings) == 0) {
                kernel_serial_settings.flags |= ASYNC_LOW_LATENCY;
                kernel_serial_settings..
                ioctl(fd, TIOCSSERIAL, &kernel_serial_settings);
    }

The port is opened, and I receive data. If however the USB device is disconnected later on, and I call the read function:

nRead = _read(fd, buffer, length);

nRead will return 0. I would expect that it should return a negative number to indicate an error (that the device is no longer available).

How do I detect that the opened device was disconnected?

Answer 1

When read() returns zero, you can call stat() on the device filename. If the USB device is disconnected, the device node has vanished, and stat() will return -1 with errno==ENOENT.